The performace difference between the dual 2.0 and the dual 2.5 isn't that big that it will have an enormous speed bumb. Unless you really need the extra power, just keep your money.
But hey, if you have too much money to spend and always want to have the newest top-of-the-line hardware, feel free and buy it

Concerning the G6, that's still years to go before it will come (if it will).

Hi guys do you think it is worth upgrading from a dual G5 2ghz to a dual G5 2.5ghz. Thanks and does anyone know when the Powerbook g5's are coming! :p

For what its worth from a new guy ... no unless you are doing some really serious cpu intensive stuff. Or like mentioned before, you have $$ burning a hole in your pocket. AND IF that is the case, look at upgrading your video card and monitor.